Field of the Invention
The present invention concerns methods and devices for magnetic resonance imaging, and in particular to such methods and devices that acquire magnetic resonance raw data from multiple slices simultaneously, a technique known as simultaneous multislice (SMS) imaging.
Description of the Prior Art
MR imaging is a widely used imaging modality for medical diagnosis as well as for material inspection.
In a magnetic resonance apparatus, the examination object (a patient, in the case of medical magnetic resonance imaging) is exposed to a strong and constant basic magnetic field, by the operation of a basic field magnet of an MR scanner, in which the examination object is situated. The MR scanner also has a gradient coil arrangement that is operated in order to activate gradient fields that spatially encode the magnetic resonance signals. The magnetic resonance signals are produced by the radiation of radio-frequency (RF) pulses from an RF radiator, such as one or more antennas, in the MR scanner. These RF pulses excite nuclear spins in the examination object, and are therefore often called excitation pulses. The excitation of the nuclear spins at an appropriate frequency gives the excited spins a magnetization that causes the nuclear spins to deviate, by an amount called the flip angle, from the alignment of the nuclear spins that was produced by the basic magnetic field. As the nuclear spins relax, while returning to alignment in the basic magnetic field, they emit MR signals (which are also RF signals), which are received by suitable RF reception antennas in the MR scanner, which may be the same or different from the RF radiator used to emit the excitation pulse.
The emitted MR signals have a signal intensity that is dependent on the exponential decay over time of the magnetization of the nuclear spins. The acquired signals are digitized so as to final raw data, which are entered into a memory that is organized as k-space, as k-space data. Many techniques are known for reconstructing an image of the examination object from the k-space data.
By appropriately selecting different characteristics of the MR data acquisition sequence that is used, the acquired signals can be differently weighted so that different sources of the detected MR signals (i.e., different tissues in the case of medical MR imaging) appear with different contrasts in the reconstructed image. In the case of medical MR imaging, a weighting is selected that causes the tissue that is important for making the intended medical diagnosis to have the best contrast (brightness) in the reconstructed image. One such type of weighting is known as T1-weighting, because it depends on the so-called T1 relaxation time of the nuclear spins.
Many different techniques are known for acquiring the raw MR data. One such technique is known as simultaneous multi-slice (SMS) acquisition, which is a technique for accelerating the acquisition of the data from a given volume of the examination object, wherein nuclear spins in multiple slices are excited simultaneously, and the resulting MR signals are simultaneously acquired from each slice. This results in a dataset in k-space that is composed of data from the multiple slices collapsed on top of each other. Techniques are known for separating or uncollapsing the data for these respective slices during image reconstruction, such as the slice GRAPPA (Generalized Autocalibration Partially Parallel Acquisitions) technique, which is schematically illustrated in FIG. 1. In the example shown in FIG. 1, multiple slices S1, S2 and S3 are excited simultaneously, resulting in each slice generating an echo train of magnetic resonance signals, which are acquired according to the known blipped CAIPIRINHA (Controlled Aliasing in Parallel Imaging Results in Higher Acceleration) technique. Details of such techniques are described, for example, in Setsompop et al., “Blipped-Controlled Aliasing in Parallel Imaging for Simultaneous Multislice Echo Planar Imaging With Reduced g-Factor Penalty,” Magnetic Resonance in Medicine, Vol. 67, pp. 1210-1224 (2012) and Setsompop et al., “Improving Diffusion MRI Using Simultaneous Multi-Slice Echo Planar Imaging,” NeuroImage, Vol. 63, pp. 569-580 (2012) and Cauley et al., “Interslice Leakage Artifact Reduction Technique for Simultaneous Multislice Acquisitions,” Magnetic Resonance in Medicine, Vol. 72, pp. 93-102 (2014).
Excitation of the nuclear spins in the simultaneously acquired slices is implemented with a multi-band (MB) RF pulse. An MB RF pulse is generated by the superimposition of a number of individual single band (SB) RF pulses, of the type that are typically used to excite nuclear spins in a single selected slice in conventional magnetic resonance imaging.
The turbo spin echo (TSE) sequence is the “clinical workhorse” sequence for MR imaging, by virtue of being the most utilized sequence for all types of body region imaging. A TSE sequence has several echo trains, and in each echo train, multiple phase encoding lines of the entirety of k-space are scanned (filled with data) after one excitation pulse. This is achieved by refocusing the spins after each readout line, utilizing refocusing RF pulses. Compared to a conventional spin echo (SE) sequence, the acquisition time in a TSE sequence is reduced by the number of refocused echoes in one echo train. This reduction is known as the turbo factor.
A conventional TSE sequence is illustrated in FIG. 2, with the example of sixteen echoes.
It is known to combine SMS and TSE, in order to acquire data from two or more slices simultaneously. This reduces the minimum repetition time (TR) which is given by the length of all echo trains for all slices that are executed back-to-back. The reduction occurs because fewer slices must be acquired with such a combination. The total number of reduced slices is known as the slice acceleration factor. For many examinations, however, the minimum TR is not limited by the total time of all echo trains, but instead is limited by the desired image contrast.
For example, for T2-weighted imaging, a long TR is necessary to allow for T1 relaxation to provide the desired T2 contrast. This means that if the TR is five seconds without SMS, an SMS factor of 2 would allow a TR reduction to 2.5 seconds, but this reduction cannot be achieved without changing the image contrast to a level that is not clinically acceptable.
The echo trains for two adjacent slices in TSE imaging are often produced in two concatenations, such as to prevent slice-crosstalk effects. With SMS it would be theoretically possible to reduce the number of concatenations, but in practice this would again lead to slice-crosstalk.
In order to provide adequate diagnostics, it is often necessary to acquire the identical slice stack of a subject with two different TSE contrasts. One example is a T2-weighted TSE and a T2-weighted TSE with CSF attenuation for the brain (known as a FLAIR sequence). Another example is a T1, proton density (PD) or T2-weighted TSE with and without fat saturation (fs) for joint imaging, known as T1/T1 fs, PD/PD fs or T2/T2 fs.
The fluid attenuated in version recovery (FLAIR) T2-weighted TSE sequence basically has a T2-weighted contrast and, in the case of brain imaging, the cerebrospinal fluid (CSF) is suppressed by preceding inversion pulses, and a relatively long weighting time (approximately 2.5 s) between the IR pulses and the readout module. A conventional T2-weighted TSE sequence with TR=5 s is shown in FIG. 3, and a conventional FLAIR T2-weighted TSE sequence with TR=8 is shown in FIG. 4. The echo train section at the right of the sequence in FIG. 4 is identical to the section shown in FIG. 2 (despite the longer inter-slice echo train gap, which is not necessary but results from the available fill time). However, inversion pulses with a waiting time have to be applied to obtain the fluid attenuated contrast.
The echo trains for two adjacent slices are often produced in two concatenations in order to prevent slice crosstalk effects. With SMS, it would be theoretically possible to reduce the number of concatenations, but in practice this would again lead to slice crosstalk. Another drawback is that the IR pulse might not be realized with SMS due to peak power limitation (two single-slice pulses summed to one multi-band pulse lead to twice the peak power).
A typical MR examination may include multiple TSE sequences with different contrasts. For example, a typical head scan includes the aforementioned T2-weighted TSE and a FLAIR T2-weighted TSE.
Another MR imaging technique for acquiring a dataset containing image data from multiple slices that allows the image dataset to have different contrasts, is described in United States Patent Application Publication No. 2015/0260820. In the procedure described therein, an MB RF pulse is radiated in a first step in order to excite nuclear spins in at least two slices of a subject. The slices are phase encoded in a second step by applying a phase encoding gradient. The scan signals of the excited slices are spread out in a third step, using each coil of a multi-coil array. The phase of the scan signal in one of the first or second steps, in at least one slice, is modulated at least once in order to cause the phase of scan signal thereof to be different from the phase of the other slices. Image datasets are reconstructed dependent on the modulation of the phase of the scan signal in at least one slice. The MB RF pulse has, in at least one sequence of the first, second and third steps in at least one slice, a different amplitude and/or duration and/or pulse shape and/or deflection angle from the other slice or slices, with the deflection angle difference being other than 180°. By varying at least one of the amplitude, duration or pulse shape in the slices, in addition to the phase shift, the image datasets have different contrasts in the images of the different slices that are reconstructed.